I tried to open a document created in Word for Windows, but it displayed the message "Permission for this document is currently restricted."
Okay, you really are stuck this time. This message means that the document is protected with Windows-only information rights management.
The only fix is to have a Windows user unprotect the document for you by opening the document in Word 2003 and choosing File » Permission » Unrestricted Access. You can't unprotect it in Word for the Mac.
Avoid the "Unable to Save" Message When Saving to a Server
My Mac connects to a Windows server so that I can share files with my coworkers. I can open a file on the server, make changes, and save it a few times, but after a while, I get an "Unable to save: Disk full or too many files are open" message. This is my only open file in Word, no other application is running except Mail, and the server has about as much free space as Alaska. Someone is lying to me!
This is a communications issue between early versions of Panther (or Jaguar, if you're still using it) and Windows and Mac servers. If possible, upgrade to Mac OS X 10.3.3 or a later version. Failing that, choose Word » Preferences, click the Save tab, uncheck the "Allow fast saves" box, and check the "Always make backup copy" box. Doing so will usually let you save 50 times or more. To stay safe, close and reopen the document periodically, rather than leaving it open for long stretches.